Submm extragalactic background light maps the history of star formation throughout the cosmic time. Since the discovery of the submm background sources, the progress in understanding their properties has mostly focused on untraluminous sources because of the confusion limit of current submm instruments. Very little was known about faint sources below the confusion limit despite the fact that they comprise most of the submm background. We break through the confusion limit with our intensive multi-wavelength surveys in the GOODS HDF-N. With very complete redshift measurements, we found that most of the submm background comes from z~1, much lower than previously thought. This result revises the current picture of cosmic star formation and baryonic mass assembly history.